Cellular communications systems continue to grow in popularity and have become an integral part of both personal and business communications. Cellular phones allow users to place and receive voice calls most anywhere they travel. Moreover, as cellular telephone technology has increased, so too has the functionality of cellular devices. For example, many cellular devices now incorporate personal digital assistant (PDA) features such as calendars, address books, task lists, etc. Moreover, such multi-function devices may also allow users to wirelessly access electronic mail (email) messages and the Internet via a cellular network.
Various cellular communications standards have been developed for cellular communications systems. One of the more prominent standards is the Global System for Mobile Communications (GSM) for digital cellular systems. To more readily accommodate new services such as email, Internet, video, etc., GSM cellular systems are gradually moving toward third generation (3G) technology. General Packet Radio Service (GPRS) is one important advancement in the migration to 3G. GPRS allows a permanent data connection and free information flow for the end user's mobile cellular communications device. GPRS also provides for a more advanced billing and charging system. That is, it allows charging based on the services that a user will access, not simply the duration of the connection.
Another advancement in the migration to 3G is the Enhanced Data Rates for Global Evolution (EDGE). EDGE will allow data speeds up to 384 kbit/s so that the advantages of GPRS may be fully utilized with fast connection set-up and higher bandwidth than traditional GSM technology.
One potential difficulty in the evolution to GPRS and EDGE is that some GSM systems may not be set up to provide the low bit error rate (BER) performance necessary for these services. Achieving high data rates at a low BER may in some cases require large scale additions of base stations, which would result in a substantial cost to a cellular service provider.
In addition, cellular communications often occur in environments where severe fading (i.e., Rayleigh fading) is encountered, which tends to cause burst bit errors. Many of the current GSM/GPRS implementations are designed for voice services, which may be more forgiving with respect to fading and burst bit errors than other services. That is, data services generally require improved error performance, which may result in lower data rates and/or increased numbers of retransmissions. As a result, throughput is decreased, which results in higher costs for the cellular service provider.
One approach for addressing the effects of Rayleigh fading is generally discussed in an article entitled “An analysis of Pilot Symbol Assisted Modulation for Rayleigh Fading Channels” by Carvers, IEEE Transaction on Vehicular Technology, vol. 40, no. 4, November 1991. Carvers discusses the use of pilot symbol assisted modulation (PSAM) to mitigate the effects of rapid fading in mobile communications applications. For PSAM, the transmitter periodically inserts known symbols, from which the receiver derives its amplitude and phase reference. While PSAM reduces the effective bit rate and introduces delay (requiring additional buffer space) at the receiver, Carvers notes that it also advantageously suppresses the error floor and enables multilevel modulation without changing the transmitted pulse shape or peak-to-average power ratio.
Despite such prior art approaches, further improvements may be desirable when implementing new services and functionality with existing GSM or other cellular systems.